Two-component polymeric materials, such as reactive adhesives, paint, gasket materials, and caulking materials comprise two separate components which when intermixed begin to react chemically with one another. For example, epoxies are a two-component material which includes a first polymeric material, such as a resin, and a second material, such as a hardener. When intermixing the two different components together, a predetermined ratio must be maintained so that the operating performance of the resulting mixture of adhesive is maintained. Changing the ratio of the mix of components, such as by adding too much of one component may undesirably alter the characteristics of the combined mixture. It is therefore important that the ratio of the components of the two-component mixing and dispensing systems be exactly maintained. This ratio is especially difficult to maintain when the resultant mixture of materials is dispensed in an intermittent operation, i.e., which is repeatedly turned on and off. In such applications, loss of ratio control may occur shortly after the dispensing valve is opened, during which time a transient imbalance phenomenon may occur. This transient imbalance may be caused by either hydraulic or mechanical elasticity of the system and the changing hydraulic pressures associated with cycling the dispenser.
In addition to the transient imbalance phenomenon described above, a loss of precise control of the flow rate of the mixture may occur as a result of changes in the flow characteristics of one or both of the components due to changes in the viscosity over time. For example, if the material is supplied from a drum, the viscosity of the fluid can vary with changes in temperature as the drum sits in a warm production area after having been moved from a cold warehouse. Viscosity may also vary from one batch of material to the next, as well as from the top of a given drum to the bottom of the same drum.
Furthermore, when dispensing viscous fluids, it is often necessary to apply the material to the surface of a workpiece in a bead containing a desired amount of material per unit length. In high production processes or where the bead of material must be positioned with accuracy, robot arms are often used to apply the material by rapidly guiding a dispensing nozzle in a programmed pattern over the surface of a workpiece. Depending on the application, the fluid being dispensed may either be projected some distance from the nozzle in a high velocity stream or extruded from the nozzle at lower velocity with the nozzle located closer to the workpiece. In either case, the amount of material applied per unit of linear distance along the bead will vary according to both the flow rate of material discharged from the dispensing nozzle and the speed of the nozzle with respect to the workpiece.
For example, in the automotive industry, it is necessary to apply a uniformed bead of material around the periphery of a panel before it is bonded to the frame of the automobile. Along straight portions of the pattern, a robot arm can move the nozzle quickly. However, where the desired bead pattern changes direction abruptly, such as around corners, the robot arm must be slowed down to achieve a required bead positioning accuracy. It can be appreciated that if the flow rate of the dispensed fluid material is held fixed, the amount of material in the applied bead will increase as the velocity of the robot arm decreases to negotiate changes in direction and will decrease as the robot arm is accelerated. Therefore, the flow rate of the material must vary with the speed of the robot arm. However, when dispensing non-newtonian fluids, the overall instantaneous viscosity of the fluid varies with shear rate in a non-linear fashion. Therefore, the shear induced by the geometry of a nozzle to the mixture being dispensed causes the pressure required to dispense the mixture to vary in a non-linear fashion with regard to the speed associated with the robot arm.
Another problem associated with two-component dispensing is that when the two materials forming the two-component mixture are brought together, they begin to cure. This cured material may tend to collect on surfaces of a mixer and/or the dispenser, particularly when such components are dispensed intermittently. The longer these materials cure, the greater the probability exists that a restriction of the flow of the materials through the mixture may occur or that a complete blockage may occur. It is therefore desirous to mix the two components together and to dispense them immediately. However, the dispenser associated with the respective robot arm, must be as compact and light weight as possible.
A dispenser mounted at the end of the robot arm cannot be too heavy otherwise it will overload the robot arm. Furthermore, a heavier mass carried at the end of the robot arm increases the difficulty of the robot arm to accelerate and decelerate in applying the bead of adhesive to various parts of a workpiece. On the other hand, remotely mounting the dispenser from the robot may further increase the transient imbalances which occur due to the elasticity of the system.